This invention relates to modems for transmitting and receiving data and, more particularly, to the transmission and reception of data at two different frequencies by using a single voltage controlled oscillator.
It is typical, particularly in transmission systems which have long transmission lines, to use signal boosting sections connected in the transmission line at periodic intervals to amplify the data transmission signal therealong. These signal boost sections typically comprise amplifiers which receive the incoming signal at its input, amplify it and supply the amplified signal to its output. Such amplifiers are unidirectional such that each boost station must have an amplifier for each transmission direction. If separate transmitting and receiving frequencies are not used, cross-talk between the amplifiers in a signal boosting station may interfere with the transmission of data in the other direction. Thus, the typical transmission system will have a head end for receiving a signal at the transmitting frequency and for converting it to a signal at the receiving frequency. Each data station connected to the transmission line will then have a transmitter modem for transmitting data at the transmitting frequency and a receiver modem for receiving data at the receiving frequency.
The prior art modems for this type of application have not been entirely satisfactory for transmitting over plural channels at one frequency and for receiving over the plural channels at a different, second frequency.